Comprehensive Analysis of the Hydrogen Bond Network Morphology and OH Stretching Vibrations in Protonated Methanol−Water Mixed Clusters, H+(MeOH)1(H2O)n (n = 1−8)

Identification of a Grotthuss proton hopping mechanism at protonated polyhedral oligomeric silsesquioxane (POSS) - water interface

The attachment and dissociation of a proton from a water molecule and the proton transfers at solid-liquid interfaces play vital roles in numerous biological, chemical processes and for the development of sustainable functional materials for energy harvesting and conversion applications. Using first-principles computational methodologies, we investigated the protonated forms of polyhedral oligomeric silsesquioxane (POSS-H⁺) interacting with water clusters (W_n, where n = 1-6) as a model to quantify the proton conducting and localization ability at solid-liquid interfaces. Successive addition of explicit water molecules to POSS-H⁺ shows that the assistance of at least three water molecules is required to dissociate the proton from POSS with the formation of an Eigen cation (H₉O₄⁺), whereas the presence of a fourth water molecule highly favors the formation of a Zundel ion (H₅O₂⁺). Reaction pathway and energy barrier analysis reveal that the formation of the Eigen cation requires significantly higher energy than the Zundel features. This confirms that the Zundel ion is destabilized and promptly converts in to Eigen ion at this interface. Moreover, we identified a Grotthuss-type mechanism for the proton transfer through a water chain close to the interface, where symmetrical and unsymmetrical arrangements of water molecules around H⁺ of protonated POSS-H⁺ are involved in the conduction of proton through water wires where successive Eigen-to-Zundel and Zundel-to-Eigen transformations are observed in quick succession.

Anharmonic Coupling Revealed by the Vibrational Spectra of Solvated Protonated Methanol: Fermi Resonance, Combination Bands, and Isotope Effect

Intriguing vibrational features of solvated protonated methanol between 2400-3800 cm^-1 are recorded by infrared predissociation spectroscopy. Positions of absorption bands corresponding to OH stretching modes are sensitive to changes in solvation environments, thus leading to changes in these vibrational features. Two anharmonic coupling mechanisms, Fermi resonance (FR) contributed by bending overtones and combination band (CB) associated with intermolecular stretching modes, are known to lead to band splitting of OH stretching fundamentals in solvated hydronium and ammonium. Theoretical analyses based on the ab initio anharmonic algorithm not only well reproduce the experimentally observed features but also elucidate the magnitudes of such couplings and the resulting interplay between these two mechanisms, which provide convincing assignments of the spectral patterns. Moreover, while the hydroxyl group plays the leading role in all the above-mentioned features, the role of the methyl group is also analyzed. Through the H/D isotope substitution, we identify overtones of the methyl-hydroxyl rocking modes and their participation in FR.

Twisted Eigen Can Induce Proton Transfer at a Hydrophobic-Hydrophilic Interface

The investigation of proton localization at a hydrophobic-hydrophilic interface is an important problem in chemical and materials sciences. In this study, protonated benzene (i.e., benzenium ion) and water clusters [BZH⁺W_n (where n = 1-6)] are selected as prototype models to understand the interfacial interactions and proton transfer mechanism between a carbonaceous surface and water molecules. The excess protons can localize in the vicinity of the hydrophobic-hydrophilic interface, and these clusters are stabilized by various kinds of noncovalent interactions. Calculations are carried out using ab initio (MP2) and density functional theory B3LYP methods to shed more light on geometries, energetics, and spectral signatures of the protonated species [H⁺(H₂O)_n] at the interfaces. These calculations revealed few low-lying isomers, which have not been reported earlier. Scrutiny of the results reveals that proton localization in the hydrophilic environment is more stable than the hydrophobic benzene π-cloud. Furthermore, the occurrence of an O-H⁺···π hydrogen bond significantly influences the O-H⁺···O interactions in the water clusters and also intensively affects the vibrational modes of the Eigen cation. Thus, the aromatic π-clouds can stabilize the Eigen cation and at the same time, a twisted form of Eigen (one O-H⁺···π → two O-H⁺···π) can enhance the proton transfer through the water chain via a Grotthuss-type mechanism. The vibrational spectra of these clusters reveal that there is a large red-shifted frequency for the O-H⁺···O, O-H⁺···π, and O-H···π modes of interaction. The energetic values and vibrational frequencies obtained from the B3LYP method are in close agreement with the MP2 level and experimental values, respectively.

A theoretical study on the size-dependence of ground-state proton transfer in phenol-ammonia clusters

Geometries and infrared (IR) spectra in the mid-IR region of phenol-(ammonia)_n (PhOH-(NH₃)_n) (n = 0-10) clusters have been studied using density functional theory (DFT) to investigate the critical number of solvent molecules necessary to promote ground-state proton transfer (GSPT). For n ≤ 8 clusters, the most stable isomer is a non-proton-transferred (non-PT) structure, and all isomers found within 1.5 kcal mol^-1 from it are also non-PT structures. For n = 9, the most stable isomer is also a non-PT structure; however, the second stable isomer is a PT structure, whose relative energy is within the experimental criterion of population (0.7 kcal mol^-1). For n = 10, the PT structure is the most stable one. We can therefore estimate that the critical size of GSPT is n = 9. This is confirmed by the fact that these calculated IR spectra are in good accordance with our previous experimental results of mid-IR spectra. It is demonstrated that characteristic changes of the ν_9a and ν₁₂ bands in the skeletal vibrational region provide clear information that the GSPT reaction has occurred. It was also found that the shortest distance between the π-ring and the solvent moiety is a good indicator of the PT reaction.

Exploration of hydrogen bond networks and potential energy surfaces of methanol clusters using a two-stage clustering algorithm

A two-stage algorithm based both on the similarity in shape and hydrogen bond network is developed to explore the potential energy surface of methanol clusters.

Ab initio and density functional theory (DFT) studies on triflic acid with water and protonated water clusters

The structure, stability and infrared spectral signatures of triflic acid (TA) with water clusters (W_n) and protonated water clusters (TAH⁺W_n, n = 1 - 6) were computed using DFT and MP2 methods. Our calculations show that a minimum of three water molecules are necessary to stabilize the dissociated zwitterionic form of TA. It can be seen from the results that there is no significant movement of protons in smaller (n = 1 and 2) and linear (n = 1 - 6) types of water clusters. Further, the geometries of TAW_n clusters first form a neutral pair (NP) to contact ion pair (CIP), then form a solvent separated ion pair (SSIP) in a water hexamer. These findings reveal that proton transfer may take place through NP to CIP and then CIP to SSIP. The calculated binding energies (BEs) of ion pair clusters is always higher than that of NP clusters (i.e., more stable than the NP). Existing excess proton linear chain clusters transfer a proton to adjacent water molecules via a Grotthuss mechanism, whereas the same isomers in the branched motifs do not conduct protons. Examination of geometrical parameters and infrared frequencies reveals hydronium ion (H₃O⁺ also called Eigen cation) formation in both TAW_n and protonated TAW_n clusters. The stability of Eigen water clusters is three times higher than that of other non-Eigen water clusters. Our study shows clearly that formation of ion pairs in TAW_n and TAH⁺W_n clusters greatly favors proton transfer to neighboring water molecules and also enhances the stability of these complexes.

Structure, stability and spectral signatures of monoprotic carborane acid-water clusters (CBW(n), where n = 1-6)

The gas phase structure, stability, spectra, and proton transfer properties of monoprotic carborane acid-water clusters [CB(11)F(m)H(11-m)(OH(2))(1)]-(H(2)O)(n) (where m = 0, 5, and 10; n = 1-6) have been calculated using density functional theory (DFT) with the Becke's three-parameter hybrid exchange functional and Lee-Yang-Parr correlation functional (B3LYP) using 6-31+G* basis set. Results reveal that Eigen cation defects are found in CBW(n) (where n = 2-6) clusters and these clusters are significantly more stable than the non-Eigen geometry. In addition to the conventional hydrogen bond (H-bond) the role of dihydrogen bond (DHB) and halogen bond (XB) in the stabilization of these clusters can be observed from the molecular graphs derived from the atoms in molecules (AIM) analysis. Spectral information shows the features of Eigen cation and proton oscillation involved in the proton transfer process. The dissociation of proton from the perfluoro derivatives with two water molecules is more favorable when compared to the other derivatives.